An Unusual Supernova May Explode Stellar Origin Theories

Brilliant and powerful supernova blasts usually herald the explosive “death” of a massive star that has burned up its necessary supply of nuclear-fusing fuel, and has collapsed either into a dense stellar corpse called a neutron star or–in the case of the most massive stars of all–into a stellar mass black hole. However, relatively small, solitary stars like our Sun die peacefully, “gently” tossing their outer gaseous layers into space, where they become beautiful multicolored objects called planetary nebulae that surround a dense little white dwarf star–which was the now-dead small star’s core. But, something very different happens when the white dwarf dwells in a binary system with a still-living companion star–and victim. In this case, the white dwarf may gravitationally sip up its sister star’s stellar material to the point that the white dwarf “goes critical”, and blasts itself to smithereens in a supernova explosion–just like the big guys. These terrible blasts, that herald the grand finale of a vampire-like white dwarf, are classified as Type Ia supernovae. In May 2019, an international team of astronomers announced that their discovery of a strange Type Ia supernova, with unusual chemical properties, may hold the elusive key to solving the nagging mystery of what triggers these violent explosions.

The discovery of the unusual supernova was made by a team of astronomers led by the Carnegie Institution’s Dr. Juna Kollmeier. The team also included Carnegie’s Dr. Nidia Morrell, Dr. Anthony Piro, Dr. Mark Phillips, and Dr. Josh Simon. Observations obtained by the Magellan Telescope, located at Carnegie’s Las Campanas Observatory in Chile, were crucial to detecting the emission of hydrogen that makes this strange supernova, named ASASSN-18tb, so distinctive.

All stars, regardless of their mass or temperature, “live” out their entire main-sequence (hydrogen-burning) “lives” by keeping a very precarious balance between two constantly warring forces–radiation pressure and gravity. The radiation pressure emitted by a star pushes all of the stellar material out and away from the star, and it keeps this enormous roiling, broiling ball of searing-hot gas bouncy against the opposing squeeze inward caused by the crush of the star’s own gravity–that relentlessly and mercilessly attempts to pull all of the stellar material inward. The radiation pressure of a star on the hydrogen-burning main sequence of the Hertzsprung-Russell Diagram of Stellar Evolution, is the result of the process of nuclear fusion, which commences with the burning of hydrogen, the lightest and most abundant atomic element in the Universe, into helium–which is the second-lightest atomic element. This process (stellar nucleosynthesis), progressively fuses increasingly heavier and heavier atomic elements out of lighter ones.

Many supernovae are triggered when a single, very massive star, has come to the end of that long stellar road after having fused its necessary supply of hydrogen fuel into heavier things. At this point, the massive star is doomed. Frequently, the supernova progenitor contains an extremely massive core that weighs-in at about 1.4 times that of our Sun (the Chandrasekhar Limit). These supernovae, that herald the death of a heavy star, are core-collapse supernovae (Type II).

Smaller, less generously endowed solitary stars, like our Sun, normally do not experience that sort of final blaze of glory. Our Sun, at this time, is a rather ordinary main-sequence star. There are eight major planets, myriad moons, and a significant number of other petite objects in orbit around our Star, which dwells in the outer suburbs of our large, star-splattered, spiral Milky Way Galaxy.

Our Sun, like all stars, will not live forever–and it is no spring chicken. Our Star is experiencing an active middle age, and it is still bouncy enough to go on fusing hydrogen in its hot core for another 5 billion years. It has already “lived” for about 4.56 billion years.

Astronomers know how our future Sun will die. When it has finally managed to fuse most of its hydrogen fuel, it will evolve into a red, swollen, and glowering, bloated red giant star. Our elderly Sun will then contain a worn-out helium core, surrounded by a shell in which there is still some lingering hydrogen that is being fused into helium. Eventually, this shell will swell outward, and our Sun’s dying heart will grow ever larger and larger as our Sun continues to age. As our Sun evolves, it will fuse its helium into the heavier atomic element, carbon–the basis of life on Earth. Our Sun will end up with a tiny, extremely hot core that produces more energy than it did when it was a vibrant main-sequence star. At this point, the outer gaseous layers will be swollen and red, and our Sun in this red giant phase will wind up devouring some of its own orbiting inner planets–first Mercury, then Venus, and then (possibly) Earth. Nevertheless, the temperature of our red giant Sun’s surface will be significantly cooler than it now is.

Sometimes the life of a stellar loner is preferable to the alternative. If our Sun had a companion star, it would become a gravitational vampire, siping up its companion’s gas until it paid for its crime by reaching critical mass–and going supernova. A Type Ia supernova would herald its explosive demise.

Type Ia supernovae are fundamental to our scientific understanding of the Universe. Their nuclear-fusing ovens are necessary for generating many of the metals around us. In the terminology used by astronomers, a “metal” is any atomic element that is heavier than helium. In addition, astronomers find Type Ia blasts extremely useful because they can be used as standard rulers to measure distances across the observable Universe. But, despite their importance, the actual trigger that sets off a Type Ia explosion has been a mystery for decades. Hence, catching them in the act is crucial.

Astronomers have long attempted to acquire detailed data at the first moments of the blasts, with the goal of figuring out how these phenomena are triggered. For the first time, they succeeded in February 2018, with the discovery of ASASSN-18bt (SN 2018oh).

ASASSN-18bt is the nearest and brightest supernova yet observed by [NASA’s Kepler Space Telescope], so it offered an excellent opportunity to test the predominant theories of supernova formation”. Dr. Ben Sharpee explained in a November 30, 2018 University of Hawai’I’s Press Release. Dr. Sharpee, who is of the University of Hawai’I’s (Manoa) Institute for Astronomy (IfA), led the discovery team with Dr. Tom Holoien of Carnegie Observatories.

“The Kepler light curve is amazing. We can probe the explosion just hours after it happened,” Dr. Sharpee added.

The team’s observations strengthened a new theory proposed by visiting IfA astronomer Dr. Maximillian Stritzinger of Aarhus University (Denmark) that there may be two differing populations of Type Ia supernovae–those that display early emission and those that do not–without the need for a victimized companion star.

“We are finding that supernovae explosions are more complicated than we previously thought, and that’s half the fun,” Dr. Sharpee added.

More Fun!

Because the brilliance of Type Ia supernovae allow them to be observed across great distances in Space and Time, they are used as cosmic mile-markers. This garnered the 2011 Nobel Prize in Physics for the discovery of the mysterious accelerated expansion of the Universe, under the influence of the dark energy–an unknown substance that may be a property of space itself.

Even though hydrogen is the most abundant atomic element in the Universe, it is rarely observed in Type Ia supernova explosions. Indeed, the lack of hydrogen is one of the defining features of this class of supernovae. Many astronomers think that this is a clue to understanding what preceded this type of fatal stellar explosion. It is also the reason why observing hydrogen emission emanating from ASASSN-18tb is so important–and surprising.

Even though it is known that Type Ia supernovae are caused by the fatal explosion of a doomed white dwarf star in a binary system, it is unknown what exactly triggers the blast–although the prevailing theory is that the trigger setting off the explosion occurs when the white dwarf star “goes critical” after having consumed a fatal amount of its companion star’s material. However, whether this is the correct theory or not has been hotly debated for years.

This is what led the research team of astronomers to start their major survey of Type Ia supernovae–called 1001AS. The search began when Dr. Kollmeier was discussing the origin of these supernovae with study co-authors Dr. Subo Dong of Peking University (China) and Dr. Doron Kushnir of the Weizmann Institute of Science (Israel), along with Weizmann Institute colleague Dr. Boaz Katz. The astronomers then devised a new theory for Type Ia supernovae that involves the violent collision of a duo of unfortunate white dwarf stars.

Recently, astronomers have detected a small number of rare Type Ia supernovae that are blanketed by large quantities of hydrogen–perhaps as much as one solar-mass. However, in several respects, ASASSN-18tb is unlike these previously observed events.

“It’s possible that the hydrogen we see when studying ASASSN-18tb is like these previous supernovae, but there are some striking differences that aren’t so easy to explain,” commented Dr. Kollmeier in a May 7, 2019 Carnegie Science Press Release.

One major difference is that, in all of the previous cases, hydrogen-blanketed Type Ia supernovae were found in youthful, star-birthing galaxies where an abundance of hydrogen-rich gas still lingers. But ASASSN-18tb is located in a galaxy that hosts old stars. Another difference is that the quantity of hydrogen gas, observed in ASASSN-18tb, is substantially less than that observed surrounding the other Type Ia supernovae-and it likely amounts to only about one-hundredth the mass of our Sun.

“One exciting possibility is that we are seeing material being stripped from the exploding white dwarf’s companion star as the supernova collides with it. If this is the case, it would be the first-ever observation of such an occurrence,” noted Dr. Anthony Piro in the May 7, 2019 Carnegie Science Press Release.

Study co-author, Dr. Josh Simon, commented in the same Carnegie Press Release that “I have been looking for this signature for a decade! We finally found it, but it’s so rare, which is an important piece of the puzzle for solving the mystery of how Type Ia supernovae originate.”

Dr. Nidia Morrell was observing that night, and she promptly reduced the data coming off the telescope and showed the findings to the team including doctoral student Ping Chen, who works on 1001AS for his thesis and Dr. Jose Luis Prieto of Universidad Diego Portales (Chile), who has been observing supernovae for many years. Chen was the first to notice that this was not a common, garden-variety spectrum. All of the team members were surprised by what they saw.

“I was shocked, and I thought to myself, ‘could this really be hydrogen?’ “, recalled Dr. Morrell in the Carnegie Press Release.

In order to discuss this intriguing observation, Dr. Morrell met with team member Dr. Mark Phillips, who is a pioneer in establishing the relationship–informally named after him–that enables Type Ia supernovae to be used as standard rulers. Dr. Phillips was convinced. “It is hydrogen you’ve found; no other possible explanation,” he told Dr. Morrell.

“This is an unconventional supernova program, but I am an unconventional observer–a theorist, in fact. It’s an extremely painful project for our team to carry out. Observing these things is like catching a knife, because by definition they get fainter and fainter with time! It’s only possible at a place like Carnegie where access to the Magellan Telescopes allow us to do time-intensive and sometimes arduous, but extremely important cosmic experiements. No pain, no gain,” Dr. Kollmeier told the press on May 7, 2019.

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